Argumentative writing is a valued genre in a range of disciplines and curricula because it requires that writers develop relationships between ideas and build a deep and multifaceted understanding of the topic. Due to the multifaceted demands, and inherent structure and conventions of argumentative writing, it is also among the most difficult to master. The aim of this masters thesis is to create a prototype of an authoring tool that can help novices (13-19 years old) of argumentative essay writing construct better arguments and improve the overall structure and linearization of their texts. Embedded within this goal is the attempt to provide a framework representative of an argumentative essay, and a derivative application that reflects current theories and practices in the instruction of argumentative writing within a user-centred design. In researching and developing the XML-based framework ArgEssML (Argumentative Essay Markup Language) and the C–SAW (Computer-Supported Argumentative Writer) application, the main aims were to explore the considerations and approaches that should be used in the design of a computer-supported authoring tool to help novices improve the structure and quality of their argumentation as well as the global structure and linearization of their texts. The intention was to provide a tool ready for experimentation, rather than a tool ready for use within a classroom setting.